AMP Program No. 4

To develop an assembly language program of DOS INT 21H using function 3CH (create file)

►     The  DOS provides two methods of file handling:

1. FCB(file control block)
2. Handle functions

►     Handle functions support hierarchical file structure

►     When the program wishes to open or create a file using handle functions it passes the address of the string specifying the file to MS – DOS.

►     Function 3Ch : create file 

This interrupt creates a new file in the designated or default directory on the designated or default disk drive.

For its occurrence AH must be loaded with 3Ch

It returns a handle* into the AX register, which can be used by the program for subsequent access to the file

The following registers are also used while creating a file(see details on pg. 22)

* A handle is a 16 bit number assigned by DOS when a file is created or opened which uniquely identifies the file.

AH = 3Ch

CX = file attribute bits

                00 = normal

                01 = read only

                02 = hidden file

                03 = system file

                DS:DX = segment : offset of file name

►     If function is successful

Carry flag = clear

AX = handle

►     If function failed

Carry flag = set

AX = error code